This invention relates to vertical centrifuges of the continuous solids discharge type having a hollow bowl with a helical conveyor therewithin for moving the solids portion of a liquid-solid slurry toward a solids discharge port.
U.S. Pat. No. 3,285,506 discloses a dry thrust bearing at the lower end of the helical conveyor. The bearing operates successfully when the bearing is fabricated of a material such as polytetrafluoroethylene (e.g., "Teflon," a trademark of E. I. duPont de Nemours and Co.), which provides some damping. However, known dry self-damping bearing materials such as Teflon have an unacceptably short service life at high temperatures.
Accordingly, at ambient separation process temperatures above 275.degree. F., a rigid bearing material, such as graphite, is required. When rigid materials are substituted, a phenomenon commonly referred to as rotor whirl develops as a result of dry friction when radial rubbing occurs in the thrust bearing. This phenomenon manifests itself as a vibration of the conveyor. As frequency of conveyor vibration approaches the resonant frequency, conveyor vibration can become violent and highly destructive; mechanical parts may be damaged or destroyed.
Conveyor vibration occurs most frequently during test operation of the centrifuge when no solid-liquid slurry is in the centrifuge. To prevent damage to the centrifuge during such operation, self-induced vibratory resonance of the conveyor must be avoided.